Subtle worldbuilding

 Players who read the rule books, campaign setting books, and lore, generally want to bring elements of that into the game. Especially those players that have purchased and read splat books that include new races, subclasses, spells and abilities. In a game like 5e, the DM has no tools for managing how the players use this new material. If a player brings in a build or character idea, the DM can only sit back and watch them play it out by succeeding on dice rolls, or ban it outright out of game. Personally, I think banning concepts from the game is kind of a dick thing to do, as it breaks the implicit agreement between players and DM, but there's no other way in 5e.

AD&D, by contrast, has a lot of subtle mechanics that let the DM maintain the integrity and versimilitude of the game while allowing the player to include what he likes. The training requirement, for example, is much lambasted by the community but what it does is force player characters to seek out NPCs in the world to continue their own growth. NPCs ground the players. Gaining magical spells is not automatic in TSR D&D, in fact there are no rules at all for how magic users gain spells. Only AD&D 1e has a list of admonishments to the DM to not give out magical spells too freely, but no guide on how to give them out at all, except as random treasure spell scrolls. This means that a magic-users spellbook is firmly in the hands of the DM. If the DM does not want to give out Wish spells, Fireball, or even Protection from Good/Evil, the magic-user will never see those spells. This actually solves the "linear fighter, quadratic wizard" problem before it ever even occurs. 

On top of the mechanical controls, it allows the DM to manage the fiction of the world. Say if the DM does not want Orcs as a race, but the player brings in a Half-Orc character. The DM can allow the player to his character while simply having no other orcs or half-orcs show up in the world.  Half-orc adventurers in AD&D can "generally pass as human", and beyond mechanical bonuses don't add anything more to the setting. The NPCs don't even have to mention the different facial features of the half-orc character, and there can be no temples to Gruumsh, no shrines, no mention of Orcs in the hills, nothing. Orcs don't have to exist in the DMs world.

Then there's ways to manage the physical features of the character. "Half-orc", in an orc-less world, could just mean an ugly person. Maybe he had an ugly dad, or his parents come from a separate tribe that the civilized folk consider "orcish". And if the player is insistent on being part of a distinct demi-human race, the NPCs can all just treat that as the rantings of a deranged lunatic.  The player can play his character, and the DM can maintain her world, and the friction between the two is lessened.

  But in 5e, there is a huge interconnectivity between a PCs race, the subclass options he chooses, the background of the character (including ideals, bonds and flaws), and the player's backstory. All of this adds elements to the DMs world that she may not be comfortable with, and the only way to manage it is to talk out of character about it and waste precious game time. Instead of subtle controls, everything becomes an open debate.